Efficiency of braking the greatest, when coupling of the tire with the surface of the road maximum. In the course of braking the tire slides on a surface, and the speed of rotation of a wheel becomes less than the speed of the car.
Fig. 11.1. An arrangement of elements of anti-blocking system of brakes on cars till 2001: 1, 11 – rotors of sensors of frequency of rotation of a forward wheel; 2 – diagnostic socket; 3 – pressure modulator; 4 – electronic control ABS block; 5 – ABS relay; 6 – delay sensor; 7 – control lamp of ABS; 8 – back sensors of frequency of rotation of back wheels; 9 – rotors of sensors of frequency of rotation of back wheels; 10,12 – sensors of frequency of rotation of a forward wheel
Fig. 11.2. An arrangement of elements of anti-blocking system of brakes on release cars since 2001: 1,3, 5, 7 – sensors of frequency of rotation of a wheel; 2 – diagnostic socket; 4 – pressure modulator; 6 – delay sensor
The anti-blocking system
(fig. 11.1, 11.2)
limits pressure created in the hydraulic drive of brakes so that the size of sliding kept about optimum. Action of this system immediate and independent for each wheel. The system immediately answers each change of coefficient of coupling of a surface and loading of the car.
The anti-blocking system interferes with blocking of wheels at sharp braking thanks to what controllability of the car, even remains at bad coupling with a paving at a rain and snow. Force of adhesion between wheels and the road is more when when braking a wheel continue to rotate. Rotation frequency sensors, on one on each wheel, measure the speed of rotation of wheels. On signals from rotation frequency sensors the electronic control unit calculates the average speed which is approximately corresponding to the speed of the movement of the car. Comparing the speed of rotation of each separate wheel to the average calculated speed, the electronic block defines a condition of slipping of each wheel and establishes what wheel is in a preblocking state.
When frequency of rotation, any of four sensors, transmits a signal of blocking of the corresponding wheel, the electronic control unit immediately gives a closing signal to the corresponding inlet electromagnetic valve which blocks supply of brake fluid via the pipeline to a wheel brake, and force of braking remains to a constant. If sliding continues, then the final valve opens, and pressure in hydraulic system of this brake decreases — the wheel is not braked. Surplus of brake fluid comes back to a tank. As soon as the wheel starts over again rotating, the inlet valve opens, and the final valve is closed. Pressure increases in a contour, and the wheel starts over again being braked.
Such cycle of braking and free rotation of a wheel when braking passes several times in a second and repeats separately for each wheel until the pedal of a brake or before reduction of speed of the car to 2–3 km/h is released.
The system of emergency shutdown provides shutdown of ABS at any malfunction or at low voltage in onboard network of the car (lower than 10 V). Malfunction of ABS does not exert impact on work of brakes, they function the same as if this system was not in the car.